Recently, agricultural tractors and machinery are equipped with hydrostatic steering devices, thus eliminating mechanical connections between the steering wheel and the steering axis. Conventionally the hydrostatic steering device is operated in the so-called open centered, hydraulic mode. A flow control valve is placed between the pressure source and a steering valve to maintain constant the volume of fluid flowing through the steering device when the speed of the drive engine varies. Any volume of fluid branched off from the pump flow and not supplied to the steering valve is fed to a further hydraulic system which is connected to a hydraulic load.
Steering valves including load sensing means are known. The priority valve to branch off the fluid flow from the pump is a pressure-compensated valve means. As long as the steering device is not operated, the pressure-compensated valve supplies the full pump flow to the hydraulic load, except a very small control fluid flow which is fed through the load sensing line to a land of the steering valve piston. When the steering device is operated, the control flow in the load sensing line is changed by said piston land so that the valve feeds a volume of flow to the steering valve corresponding to what is required for the steering operation.